The invention concerns shielding for radiation therapy source applicators, especially adjustable shielding for dynamic control of a radiation emission pattern.
Several forms of ionizing radiation therapy, particularly brachytherapy in which radiation is administered from a radiation source positioned within an anatomical cavity, are delivered using an applicator. Purposes for use of an applicator may include positioning the source within the cavity, mitigating radiation intensity incident on the cavity wall (radiation intensity decays exponentially with distance from the source), or tailoring cavity shape to facilitate radiation delivery in accordance with prescribed therapy parameters. Other purposes may also exist. The anatomical cavity may be a natural cavity, or may result from surgical intervention, for example as in the case of removal of a cancerous lesion.
Some applicators are essentially solid and of fixed configuration in that their shape doesn't vary during therapy. A typical fixed configuration applicator might comprise a catheter or wand, fashioned for placement within a body cavity, and into which a source, usually contained within a catheter, can be positioned. Such an applicator can be inserted into the body either through a natural orifice, or through a surgical incision or entry. Other forms of applicators may incorporate extensible elements which can be caused to alter shape after insertion into the body. A common form of the latter is a balloon applicator. Proxima Therapeutics Inc. of Alpharetta, Ga. (now part of Cytyc Corporation of Marlborough, Mass.) offers extensible applicators incorporating balloons. These applicator balloons are generally inflated with saline solution so as to attenuate radiation intensity near the source itself. With such an applicator, the source is ideally confined within a tube or channel within the balloon such that the position of the source within the outer skin or surface of the balloon is controlled and known. Preferably the shape of the inflated, extensible surface is coordinated with the radiation field of the source such that the intensity of radiation delivered just outside the surface of the balloon is uniform, below dangerous levels, but still strong enough to provide effective therapy.
Traditionally, the strength of the balloon, i.e. its rigidity or conformability, is chosen such that it shapes or tends to conform the tissues surrounding the cavity to the desired shape of the balloon as well to the radiation field expected from the source. When these factors can be simultaneously achieved, a uniform or isodose prescription can be delivered to the inner-most tissues forming the cavity. When this is the case, the therapist can be assured of a uniform therapy throughout the target tissue adjacent the cavity. All too often, however, this condition cannot be produced. Sometimes the cavity cannot be reshaped such that the radiation intensity outside the balloon is insufficiently uniform. Should this situation arise, a balloon which conforms to the existing cavity might be employed, but then different measures must be taken to assure delivered dose uniformity. In other instances, nearby tissue structures may lie within the therapeutic range of the radiation outside the balloon, and so would be injured were a therapeutic dose of radiation delivered. In these instances, the therapist cannot deliver a preferred treatment unless measures are taken to avoid over-treatment of at-risk tissue. It is these measures to which this invention is addressed.
The radiation source is usually positioned within or near the distal tip of a catheter to facilitate handling of the source and positioning it within the applicator. The radiation source may be isotopic in nature or it may be electronic, producing x-rays which can be utilized to produce a therapeutic effect similar to isotopes. Isotope sources pertinent to use with balloon applicators are generally referred to as high dose sources and may be in the form of point sources, comprising a single isotope “seed”, or they may be linear sources, comprising a series of seed sources, or a wire. When the source is a seed, multiple seeds or a wire, it is generally positioned in a catheter for insertion into the applicator in order to access the anatomical cavity to be treated. In other applications, the radiation source can be a fluid comprising radioactive material in suspension. This fluid can be used to inflate the extensible element of the applicator, rather than saline.
Radiation from radioisotopes is emitted in a known manner with a decaying intensity measured by the isotopes' half-life—the time at which half of their original intensity remains. Within practical time constraints, these parameters for a given radioisotope are fixed and they cannot be altered thus offering no possibilities for control. Furthermore, radioisotopes emit radiation at a few distinct energy bands, radiation from each band having its own ability to penetrate tissue and deliver dose. For example, the high-energy band of radiation emitted from 192Ir, the most common high dose-rate brachytherapy isotope, penetrates through large thicknesses of shielding materials. In addition, isotopes are always “on”, so controlling the output with on/off switching is not possible. Other common and medically relevant radioisotopes also have emission spectra containing high-energy components that make selective shielding within a body cavity impractical due to space considerations. The radiation from these isotopes will penetrate any practical thickness of shielding material. This high-energy radiation easily penetrates well beyond the target site requiring therapy, thus delivering radiation to healthy parts of the body and risks injury. It also puts the therapist at risk, necessitating “bunker” type installations within which therapy can be conducted in the absence of attending personnel. This is a major disadvantage to the use of isotopic radiation in therapy.
In contrast, with electronically controlled radiation sources, the shape of the anode and its structure, and any minimal shielding utilized, determines the directionality of the x-rays emitted. Such an x-ray source is described in U.S. Pat. No. 6,319,188, the specification of which is incorporated herein in its entirety by reference. The emitted x-rays from such a source may be emitted isotropically, they may be directed radially, axially, or a combination thereof. Anode shaping is well known by those skilled in the art of x-ray generation apparatus. Anode shape, target thickness and target configuration can be used to change the radiation profile emitted from the miniature x-ray source. Also, miniature x-ray sources capable of producing the therapeutic effects of high dose rate isotopes only require thin radiation shields to selectively block emitted radiation, thus producing a directionally shaped radiation field. With electronically produced x-rays, the acceleration voltage determines the energy spectrum of the resulting x-rays. The penetration of the x-rays in tissue is directly related to the energy of the x-rays. The cumulative radiation dose directed at a point of the lesion may be controlled by x-ray source beam current or by “on” time within the body of the patient. Control of these parameters may be applied manually, or it can be automated in real-time based on matching output to a prescribed dose based on sensor feedback to a controller. An exemplary controlled system is described in patent application Ser. No. 11/394,640, filed Mar. 31, 2006, the disclosure of which is herein incorporated by reference in its entirety. This ease of control with x-rays and their minimal safety requirements are significant advantages to the therapist and the patient.
Therefore, in order to provide the therapist the ability selectively to protect radiation-sensitive or normal tissue structures from therapeutic dosages prescribed to treat diseased tissue, convenient apparatus and methods are needed which can be adapted to selectively shield these at-risk normal tissues while allowing prescribed dosages to adjacent, diseased tissue. The apparatus and methods of this invention provide the therapist this ability.